A position-measuring device for determining the relative position of two objects movable relative to each other along a measuring direction (angular position measuring device) includes: a periodic measuring graduation configured on a ring-like material measure (i.e., on a ring-like support), e.g., in the form of a measuring strip; a scanning unit for optically scanning the measuring graduation using electromagnetic radiation (using light) which is generated in an illumination device (light source) of the scanning unit, the material measure and the scanning unit movable relative to each other along the measuring direction; a scanning plate of the scanning unit having a periodic graduation (scanning graduation), which is arranged in the beam path of the electromagnetic radiation used for scanning the measuring graduation, so that this electromagnetic radiation interacts both with the scanning graduation and with the measuring graduation; and a detector of the scanning unit whose detector surface is used for detecting the electromagnetic radiation modified by the scanning graduation and the measuring graduation, and is arranged with a period PD for detecting an intensity pattern formed by this modified electromagnetic radiation.
Period PD is selected so that, given a linear arrangement of the measuring strip along a straight line, period PD agrees with the period of the stripe pattern (Vernier pattern) which is generated by the interaction of the radiation, used for scanning the material measure, with the scanning graduation on one hand and with the measuring graduation on the other hand. This stripe pattern corresponds to a beat, caused by the combination of two periodic operations, namely, the interaction of the electromagnetic radiation with the scanning graduation on one hand, and the interaction of the electromagnetic radiation with the measuring graduation on the other hand.
The measuring principle on which the aforesaid position-measuring device is based is described, for example, in European Published Patent Application No. 1 081 457, European Published Patent Application No. 1 028 309 and European Published Patent Application No. 0 754 933.
European Published Patent Application No. 1 028 309 describes an optical position-measuring device for determining the relative position of two objects movable relative to each other along a measuring direction. It includes at least one periodic measuring graduation connected to one of the two objects, as well as a scanning unit connected to the other object and therefore movable relative to the measuring graduation along the measuring direction. The scanning unit has a light source, at least one scanning graduation, as well as a detector system in a detector plane that is made up of a plurality of radiation-sensitive detector elements arranged one after the other in the measuring direction.
If, given a position-measuring device of the type indicated at the outset, a measuring strip used as the carrier of the measuring graduation is arranged in ring-like fashion, e.g., by applying the measuring strip on a cylindrical body in the form of a drum, the position-measuring device may then be used in particular for angular measurement. Compared with a linear arrangement of the measuring strip (so that it extends in the manner of a scale along a straight line), when working with the ring-like arrangement of the measuring strip, e.g., along a circular arc, effects occur which impair the intensity of the radiation received at the detector of the scanning unit. An effect is when, given a ring-like arrangement of the measuring strip, because of the curvature of the measuring strip associated with that, the inner surface of the measuring strip is somewhat compressed and the outer surface of the measuring strip is somewhat elongated. The extent of this compression and elongation depends upon the thickness of the measuring strip, and is all the greater, the greater its thickness. For a measuring graduation provided on the outer surface of a measuring strip arranged in ring-like fashion, this effect leads to an enlargement of the measuring-graduation period, since the outer surface of the measuring strip is elongated (compared to its linear arrangement).
Conversely, when scanning a measuring strip arranged in a ring shape, using a scanning unit that is arranged at a defined distance in front of the outer surface of the measuring strip provided with the measuring graduation, a projection effect occurs which—viewed from the surface of the scanning plate—causes the distance between the individual markings (e.g., graduation marks) of the measuring graduation to seemingly become increasingly smaller toward the outer edges of the scanning plate. This is attributable to the projection of the curved outer ring surface of the measuring strip, provided with the measuring graduation, onto the planar, two-dimensional surface of the scanning plate provided with the scanning graduation.
The two aforesaid effects are contrary. However, the projection effect depends exclusively on the curvature of the measuring strip, while the elongation effect at the outer surface of the measuring strip depends both on its curvature and on its thickness. For comparatively thick measuring strips having a thickness of, for example, 300 μm, the two contrary effects may offset each other over a large range of various curvatures to the extent that the intensity pattern generated at the radiation-sensitive surface of the detector exhibits a sufficient contrast for a subsequent evaluation. For very thin measuring strips having a thickness of markedly less than 300 μm, e.g., less than 100 μm, however, the elongation effect at the outer surface of the measuring strip is only so small that it is no longer able to sufficiently compensate for the projection effect in the case of strong curvatures. Because of the poor contrast of the intensity pattern, the signal amplitude detected by the detector can become so small that a reliable evaluation for determining position is not possible.
In the following, when using the term “measuring strip,” provided it is not geared to a measuring strip of finite thickness, the intention is in each case to also include the limit case that the thickness of the measuring strip approaches zero. This limit case corresponds to a situation in which the measuring graduation is applied directly on the (cylindrical) body (in the form of a drum), which is otherwise used for accommodating the measuring strip. The elongation effect described above then plays no role. Thus no compensation of the projection effect takes place.